1. Field of the Invention
The present invention relates to a backlight comprising a plurality of light sources and to a liquid crystal display device provided with the backlight.
2. Description of the Related Art
Liquid crystal display devices are characterized, among other aspects, by thinness, low electricity consumption and high definition, and are getting more and more popular in the field of televisions, which conventionally use mainly cathode ray tubes (CRT), as the development of production methods of the liquid crystal display devices makes it possible to produce bigger screens. However, it is a problem that the contrast (dynamic range) of an image displayed by a liquid crystal display device is low compared to an image of a CRT. As a result, in recent years, a lot of efforts have been made to develop a technology allowing to increase image quality through an increase of the contrast of the liquid crystal display device.
For example, Japanese Patent Application Publication, Tokukai, No. 2002-99250 discloses a backlight comprising a plurality of illumination areas whose luminance is controllable independently. Further, a liquid crystal display device disclosed in Japanese Patent Application Publication, Tokukai, No. 2002-99250 virtually comprises a plurality of display areas corresponding to the illumination areas of the backlight, respectively. The luminance of the illumination light of each illumination area of the backlight is controlled according to how bright an image displayed in the corresponding display area of the liquid crystal display device is. In other words, with the configuration disclosed in Japanese Patent Application Publication, Tokukai, No. 2002-99250, in the illumination area corresponding to the display area in which a bright image is displayed, the luminance of the illumination light is controlled at a high level, while in the illumination area corresponding to the display area in which a dark image is displayed, the luminance of the illumination light is controlled at a low level. This allows for an expansion of the dynamic range, thus making it possible to achieve a liquid crystal display device capable of displaying a high-contrast image.
Japanese Patent Application Publication, Tokukai, No. 2002-99250 presents the use of a direct backlight 100 comprising a plurality of light sources 101 isolated in each illumination area by the partition wall 102, as shown in FIG. 8, as a backlight comprising a plurality of illumination areas. The light source 101 is a cold-cathode fluorescent ray tube, and a white LED (not shown in the drawings) used for luminance adjustment is disposed under the light source 101. Due to the white LED, the luminance ratio of the illumination light in adjoining luminous area is increased.
Further, according to Japanese Patent Application Publication, Tokukai, No. 2002-99250, because each illumination area is divided by the partition wall 102, the mutual interferences between adjoining illumination areas can be prevented, and it is possible to obtain an image with a higher definition.
However, the backlight 100 shown in FIG. 8 uses for each illumination area the light source 101 and the white LED. Because the light source 101 and the white LED have different wavelengths, a colored contour (color irregularity) occurs on the diffusion plate in the vicinity of the partition wall 102 when turning on a illumination area and turning off another illumination area adjoining to the lighted illumination area.